I just had a member of a team add another member to the team. But everywhere I can find says that shouldn't be the case and that only owners should be allowed to add members. Can anyone tell me why this would have occurred.

Also, once this member added the new person they were unable to remove them, as an owner I have that ability but he did not. I don't see any setting that would allow this to happen.

This is an issue as we are about to deploy Teams and haven't come across this issue.

It is a Public Team. This makes no sense to allow members to add other members. The same is true with Groups. And this brings up another problem I just found. When this member adds a member within Groups on SharePoint they aren't part of the same group within Teams? How does that make any sense? I had a member add another member within Teams and that added them to the group. But when they added a member to the group they don't show up as being a part of the Team. This makes no sense. What am I not understanding? Do I just need more coffee?

Because changes don't happen immediately when adding from SharePoint, there is a job on the back end that propagates the Team over. Assuming they were added to the 365 group member and not the SharePoint group which only pertains to SharePoint.

Ok so they were added to the o365 group on SharePoint. I'm not getting what you mean by 365 group member vs a SharePoint group. If a member is able to add another member then why can't they remove them as well. And why doesn't Microsoft outline this? I can't find anything that says the difference between and Private team/group vs a public team/group.

Because you can have someone just randomly remove all users or something, and you don't want people to be able to do that, but adding is usually less of an issue. I can't speak on the design but it make sense to me. I would suggest if you want to raise it, go to the Teams AMA coming up in December and ask Microsoft themselves :p

As for the SharePoint group they are less obvious, but Sharepoint has it's own permission groups you can create in advanced options that have always been there, you can still use them and they are not connected to 365 groups in any way. When someone gets added to a Office 365 group that 365 group gets added to SharePoint's actual "Members group" which is how it does permissions. SharePoint doesn't actual use Groups as it's security back end like Teams does.

Thank you for everyone's input. I just wish this had been made a little more clear within the Microsoft documentation. I was unable to find anything "concrete" on this which is very frustrating. But now I know and I'm able to adjust fire and move on. Appreciate the feedback and lesson.